chore: align variable naming across stacks

[LKasianAnsys]

Addresses #23

As part of the remote rendering epic, we will be moving ownership of VISOR variables to the server. Currently, the variable naming is different across stacks, with the frontend using 'spectrum' and the backend using 'variable'. Since the user facing term is 'variable', this PR renames 'spectrum' to 'variable' on the frontend to align the naming.

Notes:

1. This is a pure find-and-replace ('spectrum' -> 'variable'; 'Spectrum' -> 'Variable'; file names changed accordingly), with one exception (see note 2).
2. The runtime VisorSceneDetails payload included fields with the spectrum naming. Those were renamed as part of this PR, and as a result, the SCENE_DETAILS_SCHEMA_VERSION was bumped from 2 to 3.
3. The persisted state is unaffected, because it always used 'variable' naming all along. (i.e. no user-facing changes).